The present invention is related to U.S. Pat. No. 5,070,045, which is incorporated herein in its entirety. The present invention provides an improvement of the teaching of this reference.
The present invention is generally related to the art of preparation of glass-ceramic articles. It is well-known that this preparation has three main process steps:                melting a mineral glass or melting a mineral filler, which is a precursor of such a glass, containing an effective amount of nucleation agent(s). Mention can in general be made of a first step of melting vitrifiable starting materials containing said nucleation agent(s);        cooling and shaping the molten glass obtained, cooling to a temperature which is lower than the conversion domain (interval) of said glass;        crystallizing or ceramming the shaped glass by an appropriate heat treatment. In general, this third and last step of the process is carried out in two phases. The shaped glass (the glass article obtained after the second step of the process) is first of all brought to a temperature which is slightly higher than the conversion domain of said glass, so as to generate nucleation grains within it. The temperature is then increased up to a value which is high enough in order that the growth of the crystals on the grains be produced.        
During the second of said steps (cooling and shaping of the melted glass), it is desired to prevent any devitrification, any crystallization, which is synonymous with appearance of faults in the glass prepared.
On the contrary, during the third of said steps (ceramming), it is desired to crystallize said glass in a controlled manner in order to convert it into a glass-ceramic.
The characteristics of devitrification of the glass, in particular its liquidus viscosity, are critical during the second of said steps. It is known that a risk is taken of generating defective products—glass having devitrification faults, within its mass and/or on the surface—insofar as the viscosity during shaping of the glass in question is higher than its liquidus viscosity. Each shaping process necessitates that the glass be conditioned in a given viscosity range. A glass having a higher liquidus viscosity enables an easier shaping. The tolerance with respect to the existence of cold points is greater. This is all the more true because the volume of glass concerned is greater.
In the case of the glass-ceramic, the objects formed (cooktop plates, fire protection windows . . . ) are in general of quite large dimensions, and, whatever the shaping technique implemented for the shaping of the precursor glasses is (lamination, pressing or shaping from gobs, as described in the patent application FR-A-2,735,562), an increase in the liquidus viscosity of said glasses is an appreciable advantage, in terms of lowering the percentage of losses (of defective products, due to the devitrification) and of flexibility of shaping.
The present invention provides, as indicated above, an improvement of the teaching of the patent U.S. Pat. No. 5,070,045 (a teaching according to which, from a single glass of the type specified, glass-ceramics can be obtained rapidly, the predominant crystalline phase of which is a solid solution of β-quartz or of β-spodumene, and the linear thermal expansion coefficient of which is very low, even zero), an improvement which aims to minimize the defective products mentioned above. Said improvement is based on the incorporation of novel mineral glasses (or novel mineral fillers), which possess a maximum liquidus viscosity. Within them, much less faults are generated during said second step (of conjugated cooling and shaping). In an entirely surprising way, the present invention offers a command of said second step, which is much greater than that which the prior art offered, without the third step, of ceramming, being disturbed by it.
Said novel glasses of the invention can be converted into glass-ceramics in a short time (less than two hours), in deforming very little, just as those described in the patent U.S. Pat. No. 5,070,045. They possess, with respect to said glasses of said US patent, a liquidus viscosity which is significantly increased.
The following can be specified with reference to the prior art.
For many years, with reference to the third step of ceramming, and only with reference to said third step, the importance is known of incorporating, in combination, the nucleation agents TiO2 and ZrO2. On this subject, reference can be made to the article by D. R. Stewart entitled “TiO2 and ZrO2 as nucleants in a lithia aluminosilicate glass-ceramic” (pages 83–90 of “Advances in Nucleation and Crystallization in Glasses. Edited by L. L. Hench and S. W. Freiman. American Ceramic Society, Columbus, Ohio, 1971”). Said article explains the interest in incorporating said nucleating agents, TiO2 and ZrO2, in a molar ratio,
            R      ′        =                  TiO        2                    ZrO        2              ,equal to 2. That is, for this value of said ratio, that the rate of conversion of the glass into glass-ceramic is the fastest and that the average size of the crystals is the lowest, i.e. that the best transparency of the glass-ceramic is obtained. These conclusions were recalled recently by G. H. Beall and L. R. Pinckney in an article entitled “Nanophase Glass-Ceramics”, appearing in J. Am. Ceram. Soc., 82 [1], 5–16 (1999).
In the patent U.S. Pat. No. 5,070,045, the preparation, under advantageous conditions, notably in terms of rapidity, of glass-ceramic articles, is thus described. The cerammable glasses which are the starting materials, have the composition by weight below (%):                SiO2 65–70        Al2O3 18–19.8        Li2O 2.5–3.8        MgO 0.55–1.5        ZnO 1.2–2.8        BaO 0–1.4        SrO 0–1.4        with BaO+SrO 0.4–1.4        with MgO+BaO+SrO 1.1–2.3        As2O3 0–1.5        Sb2O3 0–1.5        with As2O3+Sb2O3 0.5–1.5        Na2O 0–<1        K2O 0–<1        with Na2O+K2O 0–<1        with        
                    2.8        ⁢                                  ⁢                  Li          2                ⁢        O            +              1.2        ⁢        ZnO                    5.2      ⁢                          ⁢      MgO        >  1.8                TiO2 1.8–3.2        ZrO2 1–2.5.        
Said glasses have a liquidus viscosity which is greater than 700 Pa·s (certainly greater than or equal to 600 Pa·s) and can be thermally crystallized into a glass-ceramic having a predominant crystalline phase of which is a solid solution of β-quartz or β-spodumene, and a coefficient of linear thermal expansion (20°–700° C.) of 0±3×10−7 K−1.
Said glasses contain TiO2 and ZrO2, as nucleation agents, which are active in the third step of ceramming set forth above (specified in said US patent), in the amounts mentioned above. With reference to Tables 1 and 2 of said US patent, it is noted that the weight ratio
  R  =            TiO      2              ZrO      2      
      (                  molar        ⁢                                  ⁢        ratio        ⁢                                  ⁢                  R          ′                    =                        TiO          2                          ZrO          2                      )    ,is at the maximum 1.9 (2.97) (Example 4).
In this US patent, no teaching is provided anyway on any incidence of said weight ratio (molar ratio) in the implementation of the second step of cooling and shaping set forth above (also specified in said US patent).
In such a context, the inventors have established that, surprisingly, in having said weight ratio
  R  =            TiO      2              ZrO      2      between 2.2 and 4.5, the yield of said second step of cooling and shaping is considerably improved without affecting in a notable way the implementation of the third step of ceramming.